Beam Geometry, Magnification & Sharpness

What factors affect the attenuation of X-rays?

Density, thickness, structural/atomic composition, and photon energy.

What does radiopaque mean?

A structure that strongly attenuates X-rays and appears white or light on a radiograph

Give two examples of radiopaque structures.

Metal, bone

What does radiolucent mean?

A structure that attenuates X-rays less and appears dark or black on a radiograph.

Give two examples of radiolucent structures.

Air/gas, fat

What factors influence tissue differentiation on a radiograph?

Atomic number, tissue density, and tissue thickness.

What are the characteristics of an ideal radiographic image? (6)

• Accurate depiction of the region of interest

• Minimal distortion and unintended magnification

• Sharpness with minimal blurring

• Appropriate brightness and contrast

• No avoidable artefacts

• Suitable for diagnosis

What does radiographic density mean in film-screen imaging?

The amount of blackening on the film

How is displayed brightness handled in digital radiography?

The computer can adjust displayed brightness depending on the radiation reaching the detector.

What is image contrast?

The difference in brightness between adjacent structures

What characterises a high-contrast image?

Fewer shades of grey and greater brightness differences between structures.

What characterises a low-contrast image?

Many shades of grey and smaller brightness differences between structures.

What is subject contrast?

The difference in X-ray intensities emerging from the patient.

What factors affect subject contrast?

Body part, thickness, density, atomic number, and kVp

What is radiographic contrast?

The contrast recorded by the image receptor.

What factors affect radiographic contrast?

The range of emerging X-ray intensities, kVp, and scatter

What is subjective contrast?

Contrast as perceived by the observer

What is quantum mottle?

A random, speckled or grainy appearance in an image that is a major source of image noise

Does quantum mottle represent real anatomical attenuation differences?

No. It is random image noise rather than genuine anatomical variation

How can quantum mottle be reduced?

Increase the number of photons reaching the detector

What is the relationship between mAs, photon quantity and noise?

Increasing mAs increases the number of photons, which generally reduces quantum noise.

What is the disadvantage of increasing mAs to reduce noise?

It increases radiation dose

What is beam geometry?

The spatial relationship between the X-ray source, patient/object, and image receptor

Why does beam geometry matter in projection radiography?

It influences image size, shape, sharpness, and accuracy

What dimensional image is produced by projection radiography?

A two-dimensional (2D) image of a three-dimensional (3D) object

What five main geometric factors are identified in the lecture?

1. Focal spot size

2. Source-to-image distance (SID)

3. Object-to-image distance (OID)

4. Central ray angulation

5. Cone-beam geometry.

What is image sharpness?

The ability to clearly define an edge or boundary between structures with different radiodensities

What is spatial resolution?

The ability to distinguish two closely spaced objects as separate

Why is spatial resolution important in radiography?

It helps reveal fine detail, such as subtle fractures and fine bone architecture

How is spatial frequency measured?

In line pairs per millimetre (lp/mm)

What does higher spatial frequency generally indicate in the lecture?

Smaller pixels and better spatial resolution

What does lower spatial frequency generally indicate in the lecture?

Larger pixels and lower spatial resolution

What is geometric unsharpness?

Blurring caused by the finite size of the X-ray focal spot

What is a penumbra?

The blurred or partially shaded region around an object’s edges caused by the finite focal spot

How can geometric unsharpness be reduced?

• Decrease focal spot size.

• Decrease OID

What is motion unsharpness?

Blurring caused by movement during image acquisition

What can cause motion unsharpness?

Patient movement, equipment movement, voluntary movement, and involuntary movement

How can motion unsharpness be reduced?

• Decrease exposure time.

• Immobilise the patient when appropriate.

• Improve patient cooperation.

• Ask the patient to hold their breath when appropriate

Why does magnification occur in projection radiography?

X-rays diverge from the focal spot, so an object can appear larger on the detector than its actual size

Which two distances particularly affect magnification?

Source-to-object distance (SOD) and object-to-image distance (OID)

What is SOD?

Source-to-object distance: the distance from the X-ray source to the object

What is the relationship between SID, SOD and OID?

SID = SOD + OID

What is the magnification factor (MF) formula using distances? ⭐

MF = SID ÷ SOD

What is the magnification factor formula using image and object sizes? ⭐

MF = image size ÷ object size

How do you calculate actual object size if image size and MF are known? ⭐

Object size = image size ÷ MF

What is the magnification factor of an image with a size of 15 cm when the actual object size is 12 cm?

MF = 15 ÷ 12 = 1.25

How can magnification be reduced?

Decrease OID or increase SID

An X-ray examination uses an SID of 76 inches and an OID of 3 inches. What is the SOD?

SOD = SID − OID = 76 − 3 = 73 inches.

Using SID = 76 inches and SOD = 73 inches, calculate the magnification factor.

MF = SID ÷ SOD = 76 ÷ 73 ≈ 1.04

If the image size is 12 cm and the MF is 1.04, what is the actual object size?

Object size = image size ÷ MF = 12 ÷ 1.04 ≈ 11.5 cm

What is distortion in radiography?

Unequal magnification of different parts of an object, which can affect its apparent size or shape

What can cause distortion?

• Different parts of the object being at different distances from the receptor

• Object tilt

• Incorrect alignment

• X-ray tube angulation.

How can distortion be minimised?

Use correct patient/object positioning and appropriate X-ray beam alignment

What is foreshortening?

When an object appears shorter than its true size

What is elongation?

When an object appears longer than its true size